Recently I was training with a friend of mine who was doing a set of bicep curls with dumbbells. I noticed he kept speeding up as he progressed though the set. I told him, "Slow it down, slow it down!" Before I could say another word, he responded to me with "No, I have to get fifteen, I have to get fifteen!" I waited until he finished his set before I said anything else. The following is a summarized representation of our conversation.
"What was that about? Why were you speeding through your set?
"Well, I needed to get to fifteen."
"What are you talking about? Who says you have to do fifteen reps?"
"Well, the more the better, right? If I have to suffer through this, I might as well not waste my time."
"What makes you think that more is better?"
"Well, I just figured that's how it was."
"So it's more important to do more reps than it is to concentrate on form and technique and not speeding through your sets?"
"Well... I don't know."
After that I had to explain to him that the number of reps performed is not nearly as important as the form, technique, and pacing of the exercise.